Performance and relation tot he "Visual Effects" Settings

I’m using a GTX280 on Ubuntu 8.04
I have a certain application that uses ray tracing for volume rendering. I noticed that when I set the “visual effects” (System->Preferences->Appearance) to none the frame rate drops by half compared to when the option is “Extra”. Do anyone have a reasonable explanation for this, since I expected the reverse. Is there a way through which the OS tells the card how much of work to do for the effects and how this reflects on CUDA applications? Also, If someone had compared GTX 280 performance on different operating systems please provide any information you have.